ISTANBUL — When 22 Christian refugees gathered in the basement of an apartment in Istanbul early on a recent Sunday afternoon, it was quickly clear that this was no ordinary prayer meeting. Several of them had Islamic names. There was an Abdelrahman and even a couple of Mohammads. Strangest of all, they jokingly referred to their host — one of the two Mohammads — as an irhabi. A terrorist.

If Bashir Mohammad took the joke well, it was because there was once some truth to it. Today, Mr. Mohammad, 25, has a cross on his wall and invites other recent converts to weekly Bible readings in his purple-walled living room. Less than four years ago, however, he says he fought on the front lines of the Syrian civil war for the Nusra Front, an offshoot of Al Qaeda. He is, he says, a jihadi who turned to Jesus.

It is a transition that has surprised everyone, not least of all himself. Four years ago, Mr. Mohammad tells me, “Frankly I would have slaughtered anyone who suggested it.” Not only have his beliefs changed, but his temperament has, too. Today, his wife, Hevin Rashid, confirms, with a hint of understatement, that he is “much better to be around.”

The conversion of Muslim refugees to Christianity is not a new phenomenon, particularly in majority-Christian countries. Converts sometimes stand accused of trying to enhance their chances of asylum by making it dangerous to deport them back to places with a history of Islamist persecution.

Mr. Mohammad’s particular experience, however, does not fit easily into this narrative. He lives in a majority-Muslim country, has little interest in seeking asylum in the West and treads an unlikely path followed by few former jihadis.

His is a story that began in a Kurdish part of northern Syria, Afrin, where he grew up in a Muslim family. Mr. Mohammad flirted with extremism in his teens. His cousin took him to hear jihadist preachers as a 15-year-old, and he adhered to some of the most extreme interpretations of Islam, “even the ones you haven’t heard of.” But when war broke out in Syria, after the country’s 2011 uprising, Mr. Mohammad initially joined the secular Kurdish forces in their fight for autonomy.

Mr. Mohammad’s subsequent ideological journey rarely made complete sense. But by his account, he became traumatized by the deaths he witnessed on the front line, which in turn re-energized his interest in the extremist versions of Islam that he had learned about as a teenager.

“When I saw all these dead bodies,” he said, “it made me believe all these things they said in the lectures. It made me seek the greatness of religion.” Or, at least, his violent interpretations of that religion.

When a friend invited him to defect in summer 2012 to the Nusra Front, a group that seeks to establish an extremist state, Mr. Mohammad readily agreed. As a Nusra fighter, he continued to witness extreme brutality. His colleagues executed several captives by crushing them with a bulldozer. Another prisoner was forced to drink several liters of water after his genitals were tied shut with string.

This time, however, Nusra’s propaganda made the violence seem tolerable. “They used to tell us these people were the enemies of God,” Mr. Mohammad said, “and so I looked on these executions positively.”

When I first met Mr. Mohammad, in his basement, I guessed at none of this. In fact, I was there to observe one of his guests, a Yazidi who had converted two months earlier. Mr. Mohammad seemed to be the group’s glue and behaved as though he had been born and bred a Christian.

It was Mr. Mohammad who led the first prayers and chants. (“People who have fled their homes,” began one, “God bring them safety.”) And it was he who distributed the coffee afterward. His calm poise was jogged only when his guests jokingly referred to him as the irhabi, a sobriquet that sent a sheepish smile across his youthful face.

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Mr. Mohammad wears a cross and has one on the wall of his living room, where other recent converts join him for weekly Bible readings.CreditPatrick Kingsley/The New York Times

In his previous life, however, Mr. Mohammad said he was an angry man whose temper frightened his relatives. When he briefly returned home for his family’s Kurdish New Year celebrations in March 2013, Mr. Mohammad was repulsed by what he saw as blasphemous celebrations, whose origins lay outside the Islamic tradition.

Indoctrinated by his months with Nusra, he spent his leave in isolation with Ms. Rashid, who was then his fiancée. Both she and his parents tried to persuade him not to return to the front line, but he ignored them.

But back at the front, Mr. Mohammad finally began to question Nusra’s motives. Scanning government territory through his binoculars, he says he saw Syrian government soldiers executing a line of prisoners with a bulldozer and concluded there was little difference between their behavior and that of his colleagues.

Disenchanted, he risked execution himself by deserting Nusra, and returning home to Afrin. “I went to Nusra in search of my God,” he said. “But after I saw Muslims killing Muslims, I realized there was something wrong.”

The next year, he and his wife fled the war entirely, leaving for Istanbul and joining around 2.5 million other Syrians in exile in Turkey. Still a zealous Muslim, Mr. Mohammad prayed so loudly that his upstairs neighbors complained. “They used to ask me, ‘When are you going to turn into a prophet?’” He still required Ms. Rashid to cover her hair and neck, and planned for her to wear a niqab, or full-face covering.

It was nevertheless Ms. Rashid herself who unwittingly prompted her husband’s rejection of Islam. In early 2015, she fell seriously ill. As her health worsened, Mr. Mohammad described her condition in a phone call with his cousin Ahmad — the same cousin who had taken him to jihadist lectures as a teenager. Ahmad was now living in Canada and, in a move that shocked Mr. Mohammad, had converted to Christianity.

An enthusiastic convert, Ahmad asked Mr. Mohammad to place his telephone close to Ms. Rashid, so that his prayer group could sing and pray for her health. Horrified, Mr. Mohammad initially refused, since he had been taught to find Christianity repellent. But he was also desperate, and eventually he gave in.

When Ms. Rashid improved within a few days, Mr. Mohammad ascribed it to his cousin’s intervention. Intrigued, he then began to entertain a sacrilegious thought. He asked his cousin to recommend a Christian preacher in Istanbul who might introduce him to the religion. He was put in touch with Eimad Brim, a missionary from an evangelical group based in Jordan called the Good Shepherd, who agreed to meet with him.

Mr. Brim said Mr. Mohammad was quickly persuaded by the benefits of a conversion, despite the lethal danger in which it would place him. “It was Bashir who was looking for Eimad,” said Mr. Brim, who also confirmed other parts of Mr. Mohammad’s narrative. “It was easy.”

Exactly why he sought solace in Christianity, rather than a more mainstream version of Islam, no one can quite explain. Reading the Bible, Mr. Mohammad claimed, made him calmer than reading the Quran. The churches he attended, Mr. Mohammad said, made him feel more welcome than the neighborhood mosques. In his personal view, Christian prayers were more generous than Muslim ones. But these are subjective claims, and many would reject the characterization of Islam as a less benign religion, much as they would reject Nusra’s extremist interpretation of it.

For Mr. Mohammad and Ms. Rashid, perhaps it was their dreams that sealed their conversion. As the couple began to consider leaving Islam, Ms. Rashid said she dreamed of a biblical figure who used heavenly powers to divide the waters of the sea, which Mr. Mohammad interpreted as a sign of encouragement from Jesus. Then, Mr. Mohammad himself dreamed Jesus had given him some chickpeas. The pair felt loved. “There’s a big gap between the god I used to worship and the one I worship now,” Mr. Mohammad said. “We used to worship in fear. Now everything has changed.”

For Mr. Mohammad, all this has nevertheless come at a high price. His rejection of Islam makes him a target for his fundamentalist former allies and he fears they will one day catch up with him. If they do, however, he reckons he now has the greatest protection of all.

“I trust,” he says, “in God.”

Eiad Abdullatif contributed reporting.

A version of this article appears in print on , on Page A6 of the New York edition with the headline: The Jihadi Who Turned to Jesus, Surprising None More Than Himself. Order Reprints | Today’s Paper | Subscribe